1. Field of the Invention
The invention concerns a method for generation of magnetic resonance exposures (images) of an examination subject, in which a dielectric element is positioned on the examination subject for locally influencing the B1 field distribution. The invention also concerns such a dielectric element for positioning on an examination subject for local influencing the B1 field distribution during magnetic resonance data acquisition.
2. Description of the Prior Art
Magnetic resonance tomography has become a widespread modality for acquisition of images of the inside of the body of a living examination subject. In order to acquire an image with this modality, i.e. to generate a magnetic resonance exposure of an examination subject, the body or a body part of the patient to be examined must initially be exposed to an optimally homogenous static basic magnetic field (usually designated as B0 field) that is generated by a basic field magnet of the magnetic resonance measurement device. During the acquisition of the magnetic resonance images, rapidly switched gradient fields that are generated by gradient coils are superimposed on this basic magnetic field for spatially coding the magnetic resonance signal. With a radio-frequency antenna, RF pulses of a defined field strength are radiated into the examination volume in which the examination subject is located. The magnetic flux density of these RF pulses is typically designated with B1. The pulse-shaped radio-frequency field therefore generally is called a B1 field for short. By means of these RF pulses, the nuclear spins of the atoms in the examination subject are excited such that they are displaced from their state of equilibrium, which proceeds parallel to the basic magnetic field B0, by an “excitation flip angle (also called “flip angle” for short in the following). The nuclear spins then precess in the direction of the basic magnetic field B0. The magnetic resonance signals thereby generated are acquired by radio-frequency receiving antennas. The receiving antennas can be the same antennas with which the RF pulses are radiated or separate receiving antennas. The magnetic resonance images of the examination subject are ultimately created based on the received magnetic resonance signals. Every image point in the magnetic resonance image is associated with a small body volume, known as a “voxel”, and the brightness or intensity value of each image point is linked with the signal amplitude of the magnetic resonance signal received from this voxel. The connection between a resonant radiated RF pulse with the field strength B1 and the flip angle α achieved with this is given by the equation
                    α        =                              ∫                          t              =              0                        τ                    ⁢                      γ            ·                                          B                1                            ⁡                              (                t                )                                      ·                          ⅆ              t                                                          (        1        )            wherein γ is the gyromagnetic ratio, which can be considered as a fixed material constant for most magnetic resonance examinations, and τ is the effective duration of the radio-frequency pulse. The flip angle achieved by an emitted RF pulse, and thus the strength of the magnetic resonance signal, consequently also depend (aside from the duration of the RF pulse) on the strength of the radiated B1 field. Spatial fluctuations in the field strength of the excited B1 field therefore lead to unwanted variations in the received magnetic resonance signal that can adulterate the measurement data.
In the presence of high magnetic field strengths—that are inevitable due to the necessary magnetic basic field B0 in a magnetic resonance tomography apparatus—the RF pulses disadvantageously exhibit an inhomogeneous penetration behavior in conductive and dielectric media such as, for example, tissue. This leads to the B1 field exhibiting significant variation within the measurement volume. Particularly in examinations known as ultra-intense field magnetic resonance examinations, in which modern magnetic resonance systems are used with a basic magnetic field of three Tesla or more, special measures must therefore be taken in order to achieve an optimally homogenous distribution of the transmitted RF field of the radio-frequency antenna in the entire volume.
A simpler but more effective approach to the solution of this problem is to modify the (di-)electric environment of the examination subject in a suitable manner in order to compensate unwanted inhomogeneities. For this purpose, dielectric elements with a defined dielectric constant and conductivity can be positioned in the examination volume, for example directly at the patient or on the patient. The material of these dielectric elements should exhibit an optimally high dielectric constant, preferably ∈≧50. The dielectric material thus produces a dielectric focusing. The material of the dielectric element, however, should not exhibit a conductivity that is too high because, due to the skin effect, this leads to high eddy currents, in particular in the surface region of the dielectric element, that in turn produce a shielding effect that weakens (attenuates) the dielectric focusing effect. For example, typical RF field minima can be compensated that occur in magnetic resonance examinations of a patient in the chest and abdomen region by placing such dielectric elements on the chest or abdomen, which compensate the minima by locally increasing of the penetrating radio-frequency field. Such a method is specified in U.S. Pat. No. 5,227,727. Moreover, various possibilities are specified therein for the design of suitable dielectric elements.
Distilled water with a dielectric constant of ∈≈80 and a conductivity of approximately 10 μS/cm and filled into a plastic film pouch can be used to form a simple dielectric element.
Unfortunately, the use of all of these “dielectric pillows” has the unwanted side effect that they are visible in the magnetic resonance exposures. In addition to this, due to fold-over effects the dielectric element may not be imaged within the magnetic resonance exposure at the location at which it is actually positioned in real space. Thus, for example, due to fold-over effects the pillow may be shown at the upper edge of an MR image instead of at the lower edge. This leads to the impression being created when viewing the magnetic resonance exposure that the dielectric element is located not on but rather inside the body of the patient. It is in principle possible to acquire an image by a technique known as oversampling such that the dielectric element is at the correct position. In such a case, the dielectric element can be deleted in the image processing or an image section can be selected which does not even contain the dielectric element at all. Such oversampling methods, however, are quite time-consuming and therefore prolong the measurement time.